Electric circuit-arrangement



July 28, 1953 F. DE JAGER 2,647,208

' ELECTRIC CIRCUIT-ARRANGEMENT Filed March 20, 1951 2 Sheets-Sheet 1 w Y.221- ".11. r!

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INVENTOR FRANK DE JAGER AGENT u y 8, 1953 F. DE JAGER I 2,647, 8

ELECTRIC CIRCUIT-ARRANGEMENT Filed March 20, 1951 2 Sheets-Sheet 2 vINVENTQR FRANK DE JAGER AGENT of the tubes.

hereinafter.

Patented July 28, 1953 ELECTRIC CIRCUIT-ARRANGEMENT Frank de J ager,Eindhoven, Netherlands, assignor to Hartford National Bank and TrustCompany, Hartford, Comm, as trustee Application March 20, 1951, SerialNo. 216,487 In the Netherlands April 28, 1950 11 Claims. 1

This invention relates to trigger circuit-arrangements comprising atleast two grid-controlled amplifying tubes cutting ofi each other bymutual feedback, the trigger circuit selecting one of two states ofequilibrium in accordance with a control-voltage supplied thereto. Inone state of equilibrium, one of the tubes passes cur.- rent and theother is cut off, and in the other state of equilibrium the anodecurrent conditions of the trigger are reversed.

Such trigger circuits are widely used in prac "tice. They may, forexample, be used for bilateral limitation or as a non-linear amplifierof a control-voltage supplied to a control-grid of one Across the outputcircuit of the trigger circuit is produced a rectangular voltage which,for example, is positive depending on whether the control-voltageexceeds or does not exceed a particular critical value, so that thetrigger circuit gets into one or the otherstate In reliable forms ofknown trigger circuits of the aforesaid type the difierence betweencontrol- ,voltages required to trip the trigger in different directions,in other words the response sensitiveness when using conventionalamplifying tubes, for example triodes or pentodes, is approximately 1 to2 volts. 7

The object of the present invention is toprovide improved triggercircuits of the type referred to.

I According to the invention a common part of the grid circuits of thetwo amplifying tubes comprises a grid bias supply, driving the tubesinto equal anode-current conditions irrespective- 1y of thecontrol-voltage, and a switch-pulse generator connected in series withthe said supply.

In making use of the invention a third state of equilibrium of thetrigger circuit is brought about in which either the two tubes of thetrigger-circuit are non-conductive or the two tubes pass the full anodecurrent.

Similarly to know trigger circuits, trigger circuits according to theinvention may be used for or adapted to various purposes, as will be setout In order that the invention may be readily. carried into effect,three examples will now be described in detail with reference to theaccompanying drawings, in which:

Fig. 1 shows a trigger circuit according to the invention, comprisingtwo cross-wise coupled pentodes;

. Fig. 2 shows static characteristic curves asso ciated with the circuitdepicted in Fig. 1, which indicate the relationship existing between,the switch-voltage and the output voltage. of the trigger circuit atvarious valuesof the controlvoltage;

Fig. 3 shows the circuit according to the invention used as a pulse codemodulator, and, v

Fig. 4 shows. how the circuit according to the invention may be used asa pulse selector, a pulse generator and a pulse code demodulator.

In the circuit-arrangement shown in Fig. l, the trigger comprises twopentodes l and 2 cutting off one another by crosswise direct-currentfeedback and having anode resistors 3, 4 respectively and a commonearthed cathode resistor 5. The control-grid of the pentode I isconnected to a tapping point of an ohmic voltage divider comprisingresistors 6, I which is connected on the one hand to the anode of thepentode 2 and on the other hand through a resistor 8, to a point ofnegative potential with respect to the cathodes of the tubes I. and 2.The latter point is constituted by the negative terminal of a grid biasbattery 9, the other terminal of which is earthed. In a similar manner,the control-grid of the p l tode 2 is connected to a tapping point of avoltage divider havin resistors l0 and H, one end of which is connectedto the anode of pentode I, while the other end is also connected tosistor 8. 1

In order to prevent the control-grids of the pentodes I and 2 fromassuming a positive potential relative to the associated cathodes, theanodes of grid-current limiting diodes I2 and it, the cathodes of whichare earthed, are connected to the respective control grids.

Provided the bias from the grid bias battery '9 has a suitable value,the circuit-arrangement so.

the other. If, for example, the pentode I is cut off and the pentodeZ'passes current, the latter will pass current upon va suitableincrease'in potential "of the control-grid of pentod el ljand .3 willcut off the pentode 2, a subsequent decrease in potential of thecontrol-grid of pentode l causing the trigger to resume its initialstate of equilibrium.

The trigger circuit comprises a grid bias source 0 having a value suchthat, in the absence of a control-voltage, the two trigger tubes 1, 2are operated under equal anode-current conditions. In thecircuit-arrangement shown in Fig. 1, the two pentodes l and 2 are cutoil in this third state of equlibrium. A pulse-shaped switch-voltage us,shown diagrammatically in the drawing and applied through a capacitor l5at the resistor 8 connected in series with the grid bias battery 53 issupplied, in series with the said cut-off grid bias, to terminals l4.During the positive peaks of this switch-voltage the influence of thesource of negative grid bias cutting on the two tubes is reduced to sucha degree that the trigger circuit becomes normally operative and selectsone or the other of the normal states of equilibrium in accordance witha control-voltage supplied to it.

The control-voltage for the trigger circuit might be supplied. to thecontrol-grid of one of the trigger pentodes, but this produces asometimes undesirable reaction of the voltage pulses occurring in thetrigger circuit on the source of controlvoltage. Therefore, as shown inFig. 1, the control-voltage w is supplied, as is known per se, to thecontrol-grid of a pentode 'l 0 which is connected in parallel with thepentode l and the resistor 5 included in the cathode lead thereof. Theoutput voltage of the trigger shown in Fig. 1 is taken from a tappingpoint ll of a voltage divider comprising resistors l8 and 19 which areconnected between the anode of pentode and the negative terminal of thegrid bias battery 9. The output voltage set up between the tapping ofthis voltage divider and earth is designated Us.

and is positive (for example +10 volts) or negative (for example -30volts) accordingly as the trigger circuit is in one or in the other ofthe normal states of equilibrium.

The principal elements usedjin an experimental triggercircuit-arrangement as shown, in Fig. 1 are the following.

R10 27,000 ohms.

R11 27,000 ohms.

R13 27,000 ohms.

R19 39,000 ohms.

Grid bias source: 9:150.volts. Anode voltage: 250 volts. Switch-voltage:about 50 volts.

Fig. 2 shows a number of static characteristic C'Lll'VBS holding for thecircuit shown in Fig. 1. These characteristic curves show therelationship existing between the output voltage He. and the negativegrid bias as set up at the junction of the grid resistors l and H atvarious values of the control-voltage a1 supplied to the tube 'l, asstated with the characteristic curves in question.

With a control-voltage u1=1.3 volts'a characteristic curve A wasrecorded, which is formed upon a variation of the voltage as between,say, 10 volts and 150 volts through a branch A1 and upon a, variation ofthe voltage Ho between volts and l0 volts through a branch A2. Thecharacteristic curve A thus forms a loop with branches A1 and m whichare shown in part in broken lines, since the points corresponding tothese parts do not constitute stable Working points for the circuit.

In a similar manner loop characteristic curves B and 'C are found atcontrol-voltages or" 1.1 volts and l.l8 volts respectively.

If the voltage 160 is approximately 20 volts the two trigger tubes passcurrent. At a voltage as between approximately volts and volts thetrigger tube l is cut oil and the trigger tube 2 passes current and thisstate of equilibrium occurs at the last-mentioned value of U0 with acontrol-voltage of at least -1.18 volts, for example l.3 volts, while ata control-voltage 1n of l..1'7 volts or less, for example 0.8 volt, thecharacteristic curve B or a characteristic curve designated E or F isfound, in which case, at a voltage uo of approximately l00 volts thetube l passes current and the tube 2 is out off. The series ofcharacteristic curves appeared to be satisfactorily reproducible withthe circuitarrangement shown and the trigger takes up one or the otherof the normal states of equilibrium according as the control voltage isl.1'75 volts or 10.005 volt. Consequently, the response sensitiveness isapproximately 0.01 volt.

Of this high response sensitiveness full advantage is taken if, owing tothe switch-voltage us, the voltage varies periodically between, forexample, 20 volts and l00 volts or, volts and -100 volts. As appears, ineffect, from the series of characteristic curves they embrace across-hatched area in which no stable states of equilibrium of thetrigger circuit occur.

Upon periodical variation of the bias 'LLo between the said values (1.e., 20 v. and -l00 v. or l50 v. and l00 v.), by means of a pulseshapedswitch-voltage Us, the trigger circuit appears to select in eachinstance, starting from the enforced third state of equilibrium, one ofthe two normal states of equilibrium with a'high response sensitivenessand rapidity at an instant determined by the leading edge of theswitchpulse, the state of equilibrium chosen depending on a value of theapplied control-voltage m above or below a critical value, the amplitudeof the switch-pulses playing only a very minor part in the selection.

It is advantageous for the trigger tubes l and 2 both to pass current inthe third state of equilibrium, since in this case the amplificationupon response and thus the response sensitiveness of thecircuit-arrangement is particularly favourable. The voltage uo is thuspreferably chosen to be approximately -20 volts. As an alternative,however, this voltage 1L0 may, for example, be 150 volts for theposition of rest of the circuit. In this case the two trigger tubes donot pass current in the third state of equilibrium. Upon a variation ofthe voltage Mo to about -l00 volts, the circuit again selects one of thetwo normal states of equilibrium in accordance with the control-voltageapplied, but the response sensitiveness of the trigger is slightlylower.

In the embodiment shown by way of example in Fig. 1 and in the followingembodiments, the bias voltage bringing about the third state ofequilibrium and the switch-voltage are operative across the control-gridcircuits of the'trigger tubes. The increased response s'ensitivenessaimed at may, as an alternative, be obtained by causing the saidvoltages to be operative across circuits other than the control-gridcircuits, for

example across the screen grid or suppressor grid circuits of thetrigger tubes.

In trigger circuits according to the-invention, provision should be madewith respect to the switch-pulses, that the time intervals T1 betweenthe switch-pulses T2 is sufliciently great, so that at the beginning ofa switch-pulse, it does not matter whether'one or the other trigger tubewas conductive during the preceding switchpulse. Any residua1 voltagesintroducememory effects which detract from the response-sensitiveness.In view'thereof the value of the coupling resistors 6 and ID, ifnecessary shunted by small capacitors (for exampleof micromicrofarads)must not be made excessively high at a high switch-voltage frequency(for example 60 kc.) .and parasitic capacities and inductances should beminimized.

Fig. 3 shows one embodiment of a trigger circuit, used for pulsecodemodulation. The trigger circuit comprises a hexode 20 and a triode 2|,which are cross coupled conductively in a manner similar to Fig.1through resistors 22, 23, 24 and 25/ The junction point of the resistors23 and 25 is connected through a resistor 26 to a point of stronglynegative potential with respect to the cathodes of the trigger tubes 20Similarly to Fig. 1 a rectangular switchvoltage is supplied through acapacitor 21 to the control-grids of the trigger tubes. Thisswitchvoltage is again shown diagrammatically in the drawing andconsists of switching pulses having a duration T2 and an interval T1.

The anode voltage of the trigger tube 2| is capable of cutting ofi apentode 30 by way of a voltage divider comprising resistors 28 and 29.The anode circuit of this pentode comprises an integrating networkincluding a capacitor 3| and a parallel-connected resistor 32. A signalin, for example a speech signal, to be converted in pulse codemodulation is supplied to the controlgrid circuit of a pentode 33 usedas an amplifying tube and comprising'an anode resistor 34. As will bedescribed more fully hereinafter, a voltage approximating the signal tobe transmitted occurring across the resistor 34 is set up across theintegrating network 3|, 32 of this circuit-arrangement. the two anodevoltages is utilised as a controlvoltage for the trigger circuit andsupplied, for

this purpose, to the second control-grid of the hexode 20 throughcoupling resistors 35- and 36,

to the junction point 31 of which a voltage divider comprising resistors38 and 39 is connected. The free end of the resistor 39 is connected toa point of negative potential, and the tapping point of the voltagedivider 38, 39 is connected directly to the second control-grid of thehexode 20. For the sake of simplicity, Fig. 3 shows only thoseconnections which are required for a clear understanding of theembodiment of invention; it is, for example, not shown how thescreengrids and suppressor-grids of the tubes employed are connected tothe remainder of the-circuitarrangement.

In order to explain the operation of the circuit;

shown in Fig. 3 it is'assumed that the controlvoltage supplied to thesecond control-grid of the hexode 20 exceeds a certain critical value,so that, if the trig er circuit is capable of selecting between-the twonormal states of equilibrium-At The difference between takes up thatstate of equilibrium in which the 'hexode 20 passes current and thetriode 2| is cut off. Considering the operation of the switchvoltage us,by which the two tubes of the trigger are urged into the non-conductivecondition during the interval T, it will be obvious that at the--instant t,- after which the trigger is capable of selecting one of thetwo normal states of equilibrium, the. triode 2| remains non-conductiveand the hexode 20 tends to pass current. The fpentode 30 then passes anappreciable anode 'current, owing to which an increase in voltage setupat the integration capacitor 3| is produced across the latter and thepotential of the second control-grid of the trigger hexode 20 isdecreased. Ifzthis decrease is such that the potential of the secondcontrol-grid of the hexode 20 drops below the critical value, the triodewill then pass ourrentand cut off the integrator pentode 30 at thebeginning of a next following switch pulse. In this event, no energy issupplied to the integration capacitor 3| and the available charge slowlydecreases through. the discharge resistor v32. Accordingly as the valueof the signal approximation voltage across the integration capacitor 3|is higher or lower than the signal voltage, a potential having a valuebelow or above the critical value is set up at the second control gridof the hexode 20. Upon each switch pulse, however, the trigger circuitwill select such a state ofequilibrium that any difference between thecompared voltages setup at the resistor 34 and the integration capacitor3| is counteracted and, if necessary, over-compensated, so thatirrespectively of tube characteristic curves the circuit-arrangementshown in Fig. 3 strives for a minimum divergence of the potential ofthe'sec- 0nd control-grid of the hexode 20 from the said critical valueand maximum response sensitiveness. The latter is substantiallyensuredby the negative feedback circuit provided between the output circuit andinput circuit of the trigger circuit arrangement and comprising anintegrating network, and is of particular importance for variouspractical uses.

In'the circuit shown in Fig. 3, the anode circuit of the triode includesa differentiating network connected to the anode resistor 40 andcomprising a capacitor 4| and an output resistor 42.

This'difierentiating network supplies a positive pulse whenever thetriode 2| is cut off in its conductive condition. This is only possibleat the instants designated t" in Fig. 3 at the switchvoltage us, whenthe integrator tube 30 was cut off during the preceding time intervalT2. When these positive pulses are transmitted to a receiver andsupplied after reception, if necessary through a pulse generator tocorrect the shape, theamplitude and the time of occurrence, to anintegrating network followed by a low-pass filter to restrict thequantization noise inherent with code modulation, the signal obtained atthe output of the low-pass filter substantially corresponds to thesignal to supplied to the input of the circuit shown in Fig. 3. It ispointed out that in the circuit-arrangement shown in Fig. 3 variationsof the difference voltage controlling the hexode' 20 will notpractically affect the height or the width of the pulses supplied to theintegrating network 3|, 32, and consequently the voltage across theintegration capacitor 3| provided that the switch-voltage us has sufii-.ciently steep flanks.

The itrig er circuit-arrangements described -maybe=used in the'mannerillustrated in Fig.4

'9 I to separate the. channels at the receiver end in a time-multiplexsignal transmission system by pulse code modulation, in which the signalpulses are alternately present and absent in accordance with the signalsto be transmitted. At the transmitter end, the signals to be transmittedmay be converted per channel into pulse code modulation with the use ofa circuit shown in Fig. 3 and then be assembled in time division.

The circuit-arrangement shown in Fig. .4 comprises a trigger circuitsimilar to Fig. 3, followed by an integrator tube, in the :anode circuitof which an integrating network is connected. In Fig. 4,'circuitelements similar to those of Fig. 3

of all the channels received in time multiplex to be supplied to thesaid control-grid by way of the input terminal 43, only the incomingpulses coinciding with the leadingiedges at the instants it beingsupplied to the integrator tube 30. "By providing that theswitch-voltage 1.1.5 has a recurrence frequency correspondingfwith thecycle frequency of the incoming pulses, only the pulses associated witha particular signal channel will produce corresponding pulses across thecontrolgrid of the integrator tube in thecase of suitablesynchronization of the switch-voltage relative to the incoming pulsetrains, since the state of equilibrium of the trigger circuit for thetime intervals T2 is only determined by the controlvoltage at eachpreceding instant t.

In the circuit shown in Fig. 4 there will, in generaL be no need forutilizing the high response sensitiveness, in other words theconsiderable amplification thus obtainable, but such use 'is made of theproperty of this circuit that the A sensitiveness for thecontrol-voltageis exclusively present at the time of the leading edges of theswitch-pulses. With a sufficient amplitude of the input pulses, thepresence or the absence of a signal pulse determines whether or notan'output.

pulse will occur. The amplitude of the input pulses, their shape and theexact instant of the occurrence of the centralpart of an inputpulse donot ailect the shape, the amplitude and the instant of occurrence of theoutput pulse. Referring to Fig. 4, the trigger circuit consequentlyoperates at the same time as a pulse regeneratcr to correct theshape-the amplitude and the instant of occurrence of the incomingpulses, which may be utilised to eliminate noise in pulse codemodulation receivers or in relay apparatus for pulse code modulation.

Finally, it has been found experimentally that triggercircuit-arrangements according to the invention operate in the optimummanner, if the two trigger tubes have substantially the same steepnesscharacteristic with respect to their crosswise coupling. It hasfurthermore been found that trigger circuit-arrangements of the presentkind are sometimes diihcult to handle when using variable mu tubes. 1

What I claim is:

1. A trigger-circuit arrangement comprising first and second electrondischarge tubes each .havinga cathode, a grid'andan anode, means,

cross-coupling said tubes whereby said arrangement exhibits two normalstates f equilibrium, in one state of which said first tube isconductive and said second tube non-conductive, in the other stateofwhich the converse relationship exists, means to apply a control voltageto one of said tubes. to flip said arrangement from one normalequilibrium state to another, and a common grid circuit coupled to thegrids of said tubes, said common circuit including a bias voltage sourcehaving .a value at which saidtubes are both driven into equal anodecurrent conditions representing a; third state of equilibrium, and apulse generator connected in series with said source and producingrectangular switching pulses having an amplitude and polarity at whichsaid trigger arrangement alternates between said third state and onecharge tube connected across said first tube, the

control voltage being applied to the grid of said third tubea 5. Atrigger-circuit arrangement, as set forth in claim -l,wherein saidcontrol voltage is constituted by periodic pulses having a repetitionrate whichis a harmonic of the repetition rate of said switching pulses.

'6. A trigger-circuit arrangement, as set forth in -claim l, whereinsaid means to cross-couple said tubes includes a pair of voltagedividers each connected between the anode and cathode of a respectivetube, a tap on each divider being connected to the grid of thetubeassociated with the other divider.

.7. A trigger-circuit arrangement, as set forth in claim 1, furtherincluding a voltage divider connected between the anode of one of saidtubes and through said bias voltage source to the oathode thereof, andan output circuit coupled to a tap on said divider.

8. A trigger-circuit arrangement comprising first andsecond electrondischarge tubes each having a cathode, a grid and an anode, meanscross-coupling said tubes whereby said arrangement exhibits two normalstates of equilibrium, in one state of-which said first tube isconductive and said second tube is non-conductive, and in the otherstate of which the converse relationship exists, an input circuitcoupled to the grid of said first tube, an output circuit coupled to theanode of said second tube, means to apply a control voltage to saidinput circuit to flip said arrangement from one normal equilibrium stateto another, a common grid circuit coupled to the grids of said tubes andincluding a bias voltage source having a 'value at which both tubes aredriven into equal anode current conditions representing a third state ofequilibrium and a pulse generator connected in series with said sourceand producing rectangular switching pulses by which said triggerarrangement alternates between said third state and one of said normalstates.

9. An arrangement, as set forth in claim 8, further including a negativefeedback voltage cir- 9 1G 'cuit coupled between said output circuit andsaid wherein said output circuit includes a differentiinput circuit andincluding an integrating netating network to yield code modulationpulses. work. FRANK DE JAGER.

10. An arrangement, as set forth in claim 8, I wherein said controlvoltage varies in accordance 5 References Cited in the fi e O this P tet with a signal to be converted into pulse code UNITED STATES PATENTSmodulation, and wherein pulses are derived from N b N said outputcircuit. um er ate 11. An arrangement, as set forth in claim 10, 2555999Ringlee June 1951

